1. Field of the Invention
The invention is concerned with security threads for security articles such as banknotes, cheques and the like.
2. The Prior Art
It is widely to known to use in banknotes security strips or threads which are made from a transparent film provided with a continuous reflective metal layer, vacuum deposited aluminium on polyester film being the commonest example. Banknotes made from such paper have been in general circulation in many countries for many years. When such security devices are embedded in a security paper and the paper is subsequently printed to provide the security document, e.g. a banknote, the thread cannot be readily discerned in reflected light but is immediately apparent as a dark image when the document is viewed in transmitted light. Such threads are extremely effective against counterfeiting by printing or photocopying, since the optically variable effect which is provided cannot be accurately simulated, for example by printing a line on the paper.
The composition, size and positioning of security threads currently in use vary depending on the desired security of the document. Typical security threads are composed of a polymeric film such as polyester, which may be metalised or coloured and may include microprinted lettering denoting a title or message. The lettering can be produced by printing onto the substrate or by de-metallising a metallic layer on the substrate. One example of microprinted thread can be found in British patent specification No. 1095286. The threads can be slit to produce either registered lettering with respect to the edge of the thread, or unregistered lettering designed so that the message always appears in respect of thread slitting.
The widths of threads typically used vary from 0.5 mm to 2 mm and may have thicknesses typically ranging from 12 microns up to 50 microns.
The positioning of the thread within the document may be strictly controlled to agreed criteria and may be concurrent with additional security features, such as watermarks. Security threads may be fully embedded within security paper in such a way that paper fibres cover both sides of the thread, making it considerably less visible in reflective light, but clearly visible in transmitted light.
In recent times, however, in order to enhance security documents against modern counterfeiting techniques making use of sophisticated colour separation, printing and colour photocopy technology, it has become common to use a security thread comprising a thin layer of aluminium on a plastic support which is exposed on one side of the sheet at intervals along the length of the thread, the region of exposure being referred to as a window. British Patent Specification Nos. 1552853 and 1604463 disclose banknotes containing such windows. Paper for use in producing such banknotes can be made using the method disclosed in European Patent Specification No. 0059056. The dimensions of the windows typically used are from 3 mm to 14 mm lengthways, with bridges ranging from 4 mm to 30 mm therebetween. Again, the positioning of the windows may be controlled to allow registration of the window with respect of the document and other security features such as watermarks.
This latter development has resulted in enhanced security and windowed paper has been used for banknotes by many countries. A banknote of this type provides added security against counterfeiters as, when viewed in transmitted light, the strip is seen as a dark line and when viewed in reflected light on the appropriate side, the bright shining aluminium portions which are exposed at the windows are readily visible. However, there is a need for even greater security by the use of more sophisticated security devices in order to render the task of a would-be counterfeiter more difficult as the reflected light appearance of the exposed aluminium portions of a security device can be simulated to a degree by modern materials and techniques, for example by the use of hot foil stamping.
Other threads in use can be coated with protective lacquers and these may contain, for example, fluorescent inks which are only visible on illumination with ultra violet light of a specific wavelength, e.g. 366 nm.
It is also possible to make threads that are machine readable by incorporating into or onto the substrate, substances which can be identified by a detector, such as magnetic material.
Other known threads can be a single layer or multiple layers bonded or laminated together, and in some instances each layer may be coated with metal such as aluminium, stainless steel, tin and/or metal oxide such as tin oxide. The coating may be protected with the lacquer which may also contain coloured or invisible fluorescent pigment, for example rare earth chelates, such as europium acetylacetonate and the like.